The present invention relates to semiconductor memory devices, and more particularly to a method of forming a bit line of a semiconductor memory device, which can reduce the resistance of the bit line.
Recently, the research into sub-80 nm devices has been conducted. In sub-80 nm technology, forming patterns using a photolithography poses a difficult challenge. In other words, the exposure technique employing a krypton fluoride (KrF, 248 nm) light source and a Reactive Ion Etch (RIE) technique for 100 nm devices appears to have reached its limit.
In the case of 100 nm devices (e.g., DRAM), a photolithography process is used to form the critical layers using a KrF light source. For the metal layers, most of the patterns are formed using RIE. These process techniques have been used in mass production so far without significant problems. However, in the case of sub-80 nm devices, a pattern formation process performed using the existing KrF & RIE methods may not be possible.
In particular, in the case of a bit line having the smallest pitch in the design rule for 100 nm devices, the pitch becomes lower than the Final Inspection Critical Dimension (FICD) in sub-80 nm devices. In this situation, KrF has reached its limit. Thus, an argon fluoride (ArF, 193 nm) light source, which has a smaller wavelength, is being considered for the next generation device.
Due to the importance of the photo resist, a significant amount of research has been conducted on the use of ArF. This is because the ArF light source has high resolutions due to its small wavelength, but must use a thin resist to keep the Depth Of Focus (DOF) margin small. However, the ArF resist has a low etch selectivity for the resist when the RIE etch process is performed. Thus, it is expected that there will be difficulties in performing the etch process. That is, if a thin resist with low selectivity is used, etch process margins can be reduced significantly. However, the etch process for future nanoscale devices may have limitations.
To increase the etch process margins between the ArF light source and the resist, research has been conducted into a damascene process of patterning the oxide layer, depositing metal, and then performing a Chemical-Mechanical Polishing (CMP). This is because, in general, it is advantageous to etch the oxide layer rather than the metal or a nitride layer since the selectivity is high in the same resist thickness. That is, the damascene process may become a fundamental method used for increasing the process margin of the etch process. Furthermore, the damascene process can be applied to fabrication of micro devices because the process margin with respect to the photo resist becomes high in the photo process.
Furthermore, as devices are miniaturized, polysilicon and tungsten silicide are being replaced with a material having less resistivity, e.g., tungsten, in order to enhance the speed of the device. This move to metal with lower resistivity is generally applied first to the bit line since it tends to have the smallest pitch in the device.